The present invention provides a reflective surface which is capable of converting polarization of a radio frequency signal, such as microwave signal, between linear and circular, for use in various antenna applications.
The polarization converting reflector of the present invention is based on a Hi-Z surface, in which the electromagnetic surface impedance is controlled differently in two orthogonal directions by appropriately distributing resonant LC circuits on a conducting sheet. In accordance with the present invention, the surface impedance xe2x80x98seenxe2x80x99 by an incoming wave or by adjacent antenna elements is different along two orthogonal axes of the surface. For an incoming wave with linear polarization, the reflection phase depends on the angle of the polarization with respect to the two axes of the surface. In the polarization converting reflector, polarization phase is designed to differ by xcfx80/2 to for the two orthogonal directions. A wave which is linearly polarized at 45 degrees with respect the two axes is converted into a circularly polarized wave upon reflection. Similarly, and incoming circularly polarized wave is converted into a linearly polarized and wave upon reflection. Furthermore, both right-hand and left-hand circular polarization can be produced from orthogonal linearly polarized waves. When used as a reflector for an antenna, this surface is capable of collecting a circularly polarized beam from a satellite and focusing it onto a linearly polarized detector. This surface may also be used as a ground plane for a phased array having individual antenna elements comprised of straight wires, yet the array is capable of radiating a circularly polarized radio frequency signal because of the presence of the polarization converting reflecting surface disclosed herein.
The concept of using a resonant structures to convert between linear and circular polarization is not new. An array consisting of pairs of orthogonal dipoles having slightly different resonant frequencies has been disclosed by Gonzolez et. al. (U.S. Pat. No. 4,905,014). By designing the dipoles such that the reflection phase differs by xcfx80/2, the same polarization converting effect can be achieved. However, this structure requires the presence of a separate ground plane, which must be one-quarter wavelength behind the dipoles. Depending on the operating frequency, this could lead to a rather thick structure, which may be unacceptable for some applications. The present invention is much better, on the order of one-tenth of the wavelength or less. Furthermore, the Gonzolez asserts that the device only has a bandwidth of 3 percent to 10 percent using his dipole design. With the present invention, experimental data suggest a bandwidth of 10 to 20% of the center frequency of interest should be achievable.
The present invention also supersedes several current techniques for transmitting and receiving in circular polarization. By converting between circular and linear polarization, this reflector eliminates the need for a circularly polarized detector. A simpler detector having linear polarization can be used instead. Furthermore, this invention has advantages for circularly polarized phased arrays. In general, antenna elements which radiate or receive in circular polarization tend cover a large area, while linear elements can be thin, wire dipoles. Since narrow wire elements use very little area on the surface of the array, adjacent elements can be separated by a large distance. This can be used to improve isolation and eliminate the phase error that results from inter-element interaction.
A polarization converting dipole reflector, disclosed by Gonzolez et al., is shown in FIG. 1. It consists of pairs of dipoles, oriented orthogonally with respect to each other. The dipoles have slightly different resonant frequencies, and are designed so that they reflect with a phase difference of xcfx80/2 between the two orientations. If a wave impinges one of the dipoles with linear polarization, oriented at 45 degrees with respect to the other dipole, it will have circular polarization after reflection. This is due to the fact that the component oriented along one dipole is delayed with respect the compliment oriented along the other dipole by one-quarter cycle.
The Hi-Z surface, which is the subject of a provisional patent application filed by Sievenpiper et al (U.S. Ser. No. 60/079,953, filed on Mar. 30, 1998), provides a means of artificially controlling the impedance of the conducting surface by covering it with a periodic texture consisting of resonant LC circuits. These resonant LC circuits can be easily fabricated using printed circuit board technology, so the resulting structure is thin and inexpensive to build. At the resonant frequency, the structure can transform a low-impedance metal sheet into a high-impedance surface, allowing very thin antennas (having a thickness  less than  less than xcex) to be mounted directly adjacent to it without being shorted out.
The Hi-Z surface typically consists of a pattern of small (having a size  less than  less than xcex in a direction parallel to the major surface which they define) flat metallic elements protruding from a flat metal sheet. They resemble thumbtacks, or flat mushrooms, arranged in a lattice or array on the metal surface, and can be fabricated in a single or multi-layer geometry. They are usually constructed as flat metal patches, each connected to the ground plane by a via, which is drilled through the circuit board substrate material and plated with metal. The proximity of the neighboring metal patches provides capacitance C, while the long conducting path between them provides inductance L. At the resonant frequency,       ω    =          1              LC              ,
this surface exhibits high impedance. Any desired surface impedance can be achieved simply by tuning the resonant frequency. An example of a Hi-Z surface is shown in FIG. 2a along with the measured reflection phase as a function of frequency in FIG. 2b.